Abstract. Segmentation of magnetic resonance imaging (MRI) data entails assigning tissue class labels to voxels. The primary source of segmentation error is the partial volume effect (PVE) which occurs most often with low resolution imaging – With large voxels, the probability of a voxel containing multiple tissue classes increases. Although the PVE problem has not been solved, the first stage entails correctly identifying PVE voxels. We employ rough sets to identify them automatically.